The search for hydrocarbons is being pursued on a worldwide scale which includes most of the potentially prospective, water-covered, sedimentary basins. Many of these water-covered areas represent relatively new, unexplored basins where little is now known about the sedimentary section. The present state of seismic technology permits extraction of stacking velocities from seismic reflection data which, when converted to interval velocities, can under certain conditions be related in a gross way to rock types. In other words, there are occasions when it is possible to associate seismic velocities with the rock types which make up the sedimentary section. However, the accuracy of these velocity measurements is critically dependent upon a number of factors, some of which are listed as follows:
1. Seismic reflection data quality. PA0 2. High Signal-to-noise ratio. PA0 3. Known Water bottom geometry topography. PA0 4. Subsurface or below bottom structural geometry. PA0 5. Statics or maintaining the geophones in a horizontal plane. PA0 6. Accuracy of T.sub.o i.e., the instant the acoustical energy was released, measurements derived from fitting hyperbolae to the reflection data.
The present procedure for recording offshore marine seismic reflection data includes a seismic source towed along side of or immediately behind the recording boat and a streamer geophone cable which is usually a mile or more in length, made up of 24 to 96 spaced geophone groups and with the geophone group located nearest the boat positioned 200 to 500 meters behind the towed seismic source. This arrangement introduces a significant in-line offset between source to first receiver. When recording reflection data this offset can produce a degree of uncertainty in the determined T.sub.o 's used in the velocity calculations. Furthermore, towing the seismic source and geophone cable close to the boat introduces undesirable source generated noise and boat generated noise in the seismic data, particularly on those critically located, short range geophone groups.
A new mechanism for reducing the source-to-receiver in-line offset and simultaneously for moving the seismic source away from the recording boat is disclosed below.
While an offshore seismic exploration method is disclosed in assignee's prior U.S. Pat. No. 3,774,021 of July 3, 1973, involving simultaneous running of a deep-reflection profile and a shallow-reflection profile without substantial interference of one with the other and with no steerable paravane disclosed, this invention disclosed hereinafter pertains to two modifications of a marine seismic source two system for maintaining each seismic air gun spaced from the towed submerged geophones at an exact predetermined distance for producing more accurate seismic velocity measurements.
Also, while two weighting devices are illustrated for positioning a string of geophones on a cable in U.S. Pat. No. 3,187,831, no controllable paravanes are disclosed responsive to distance measuring signals. Another method of seismic exploration utilizing paravanes which will hold a preset depth is disclosed in U.S. Pat. No. 3,331,050.
A new method is disclosed for reducing the source-to-receiver in-line offset between the seismic source and first receiver, which offset can produce a degree of uncertainty in the determined T.sub.o 's used in the velocity calculations and simultaneously for moving the seismic source away from the recording boat.